Abstract: Laboratory measurements of the mechanical stiffness, hydraulic conductivity and seismic properties of three natural fractures were made as a function of applied normal stress. Seismic measurements were made under both dry and saturated conditions. Results showed that an empirical and theoretical correlation exists between seismic properties of a fracture, its hydraulic conductivity and mechanical stiffness, both the hydraulic and seismic properties could be ranked according to the mechanical stiffness of the fractures. By treating a fracture as a displacement discontinuity, quantitative relationships are developed between the mechanical stiffness of a single fracture and the seismic impedance of the intact rock, and the group time delay and amplitudes of the reflected and transmitted waves. For saturated conditions, additional relationships are developed between the group time delay and amplitudes of shear waves and the specific viscosity of a fracture.